This invention relates to a liquid level detection system and especially to a liquid level detection system in which optical fibers are used to detect the presence or absence of fluids.
Conventionally, the level of a liquid in a vessel is detected by using a float with a mechanical or magnetic coupling to an external gauge, an ultrasonic or optical transducer which measures time of flight to deduce the liquid level, or a parallel wire capacitance sensor which monitors the change in the dielectric constant between the wires associated with a change in liquid level.
The application of fiber optics to level sensing in liquids is well documented. The principal advantages of this type of level sensor are its passivity, i.e. no moving or mechanical parts, and its intrinsic dielectric properties which mean no spark hazard when used with combustible fluids and virtually zero susceptibility to electromagnetic interference. The more common implementations use small prisms mounted at the end of two optical fibers, a conically shaped optical fiber tip, or a U-shaped bent optical fiber. In all instances, the optical fiber probe or sensor is suspended or made to protrude into the vessel, and the potentially fragile sensor is exposed to damage by floating debris, vibration and dynamic effects during filling. The potential for damage is increased if routine maintenance of the sensor is required due to biological or chemical fouling of the optical surface.
The fiber optic sensors just described are primarily for discrete level measurement, e.g. to sense whether the vessel is empty or full or at some intermediate point. A multiplicity of such point sensors generally represents an impractical configuration for a continuous liquid level measure. A continuous measure is desirable, however, for improved resolution in many applications. Consider the case of an aircraft fuel gauge. The dielectric properties of the optical fiber sensor are desirable from the point of view of safety with respect to spark hazard and lightning strikes but high resolution and accuracy are also desirable so that excess fuel quantities would not have to be carried thus reducing aircraft weight and consequently fuel consumption. Present day aircraft fuel level sensors are for the most part capacitance type sensors which lose accuracy when the fuel becomes laden with water and the dielectric constant is changed significantly.
One prior art continuous fiber optic liquid level sensor is based on the bending or cladding loss principle, it consists of large loops of a single fiber, the loops being of ever increasing diameter, which are suspended in the liquid.
Another such sensor teaches a fluorescent doped detector fiber to collect light reflected from a source fiber in the presence of air. The light is refracted away when a fluid of higher refractive index. Hence, the output signal is analogous to the fluid level.
Yet another such sensor can be seen in U.S. Pat. No. 4,942,306 and uses two optical fibers, a source is transmitted into an optical fiber having one end adapted to be optically connected to an external light source, and a detector or receive optical fiber having one end adapted to be optically connected to an external light detector. The source optical fiber has at least the other end thereof embedded in an optically clear substrate material or window that is in contact with the liquid to be sensed such that the light exiting the source fiber is incident on the substrate to liquid or air interface at an angle between the critical angle for the liquid and the critical angle for air. The substrate or window material has a refractive index equal or nearly equal to that of the core of the optical fiber so that the maximum refractive index mismatch will occur at the interface to the liquid. A fluorescent detector fiber is mounted within the substrate so that it can receive any light from the source which is internally reflected from the interface. Hence, a received light signal will occur at the external light detector only when a portion of the sensor is exposed to air.
A fiber optic liquid level detector uses optical fibers to detect the presence or absence of liquids. The waveguide properties of optical fibers tend to deviate from the normal at dielectric interfaces so that a fiber optic probe whose tip is immersed in a liquid has a reflection coefficient smaller than when surrounded by air or vacuum. This is caused by the differences in refractive indices of liquid and air and is used to measure the amount of light transmitted or reflected by the fiber in the presence or absence of a liquid. A fiber optic coupler is connected to a light source from a first fiber optic line and is connected to a light detector from a second fiber optic line. A fiber optic probe is connected to the fiber optic coupler for insertion into a liquid to be detected, where only the unconnected end of the probe is sensitive to the presence or absence of fluid. A signal processor is connected to the light detector for determining the deviation of the reflected light at the dielectric interface between the liquid being detected and the atmosphere adjacent the liquid. Liquid level is measured by detecting the presence or absence of liquid, resulting from a chancre in reflective coefficient(s), using one or more fiber optic probes positioned at specific levels. A plurality of fiber optic probes are utilized for measuring a plurality of fluid levels and can be coupled to a plurality of fiber optic couplers. The signal processor can be coupled to an LED display or other displays or devices for displaying a liquid level, and/or for initiating a process event or events. Said signal processor may be a comparator circuit to compare the detector output to any desired threshold value to initiate such aforementioned displays and/or process events.